1. Field of the Invention
This invention relates to differential pulse code modulation (DPCM) apparatus, and to methods of establishing the transfer function of a compressor in such apparatus.
2. Description of the PrIor Art
DPCM offers the possibility of data rate reduction, when storing or transmitting a video signal. For example, a proposed digital slow motion processor is required to store several seconds of a video signal in random access memory. This involves a very large amount of data and requires a correspondingly large amount of random access memory. If the number of bits required to represent each picture element (pel) of the video signal could be reduced, without unacceptable degradation in the quality of the picture derIved from the reproduced video signal, then the amount of random access memory required could be reduced or the time duration of the stored video signal could be increased.
FIG. 1 of the accompanying drawings is a block diagram of a DPCM apparatus for processing an input video signal X(i), already in pulse code modulated (PCM) form, to produce a DPCM output video signal TX for storage (or transmission). The input video signal X(i) comprises successive digital words, in this example 8-bit words, which represent successive samples and are obtained by sampling and pulse code modulating an analogue video signal. It is assumed that the bits of each word arrIve in parallel and are processed in parallel within the apparatus of FIG. 1. Accordingly, it is to be understood that the elements shown in FIG. 1 (and in the subsequent FIGS.) are, where approprIate, connected by multi-bit buses or highways.
DPCM relies on accurate prediction of each input sample of the input video signal X(i), based on one or more samples that have been previously received. (Some prediction schemes suitable for a video signal are described below.) A predicted value of each input sample is subtracted from the input sample and the resultant dIfference or error signal E(i) is compressed, and then stored or transmitted. A predicted value signal X(p), comprising successive predicted sample values which are to be subtracted from successive input samples is obtained from a predictor 1 by expanding the compressed error signal E(i), and adding the result to the predicted value signal X(p). More specifically, the predicted value sIgnal X(p) is subtracted from the input video signal X(i) in an error or difference signal generating means, which is in the form of a two-input adder 2 arranged to act as a subtractor, to produce the error signal E(i) which comprises a sequence of words each representing the error or difference between an input sample word of the input video signal X(i) and a predicted value of that input sample word. The error signal E(i) is compressed by a compressor 3 to words of fewer bits to form the output video signal TX that can be stored or transmitted. The output video signal TX is also passed to an expander 4, which simulates an expander provided in apparatus for receiving the output video signal TX, so as to produce a received vIdeo signal RX. The received video signal RX is supplied to one input of a two-input adder 5. The output of the adder 5, at which appears a received error sIgnal X(o), is connected to an input of the predictor 1. The predictor 1 produces the predicted value signal X(p), which is supplied to the adder 2 so as to be subtracted from the input video signal X(i), and is also supplied to the other input of the adder 5 so as to be added lo the received video signal RX to produce the received error signal X(o).
The apparatus of FIG. 1 further comprises a clock pulse generator (not shown) which causes the above sequence of operatIons to be performed during each of a plurality of successive clock periods equal to the time spacing T of successive input sample words of the input video signal X(i).
As shown in FIG. 2 of the accompanying drawings, which is a block diagram of a modified DPCM apparatus for processing an input video signal X(i), the compressor 3 and the expander 4 of the apparatus of FIG. 1 may be implemented together in the form of a compander 11 which compresses and expands the error signal E(i) to produce the received video signal RX. This modified apparatus also requires a separate compressor 12, having characteristics similar to the compressor 3 of FIG. 1, for producing the output video signal TX.
In the apparatus of FIG. 2, one of the most important features is the characteristic of the compander 11, which characteristic is of course repeated in the combined effect of the compressor 12 and the associated expander in the receiving apparatus.
Previous work on the transfer function of a compander such as the compander 11 of FIG. 2, has put most of the emphasis on the statistical behaviour of the error signal E(i). By using standard test pictures and data logging equipment, it has been possible to build up probability maps of the error signal E(i). Using these maps companders have been desIgned which have small quantization steps for high probability errors and progressively increasing quantization steps for decreasing probability errors. ThIs technique allows the quantization noise to be filtered statistically giving pleasing results for the standard test pictures which have been used to generate the probability maps.
However, In a practical system, the video sIgnal represents a picture the spectral content of which is substantially unpredictable, and therefore a compander having a transfer function based on the spectral content of standard test pictures does not necessarily give a particularly good result.